1. Field of the Invention
This invention relates in general to methods and apparatus for refining and deodorizing edible oils and fats and particularly to methods and apparatus for continuous vacuum refining and deodorizing of fats and oils.
2. Description of the Prior Art
In the process of refining either vegetable or animal fats and oils to prepare foods such as margarine and cooking oils, several processes are presently available. Generally, these processes utilize vacuum distillation to remove odoriferous substances such as free fatty acids from the oils and fats comprising mainly triglycerides. By this means, the oils and fats are made more palatable and their odors and tastes are improved.
While the odoriferous and taste giving materials are more volatile than the oils in which they are found, the oils and fats may not be subjected to relatively high temperatures since undesirable polymerization or production of additional fatty acids, alcohols, aldehydes, etc., from the fats and oils occur at higher temperatures. Accordingly, the preferred method of deodorizing fats and oils has been by means of a vacuum distillation allowing a lower temperature removal of these more volatile odoriferous substances. Most of these vacuum processes use a stripping agent such as steam to increase the surface area of liquid oil and fat, to provide a carrying medium for removal of the volatile substances and to react with and strip certain undesirable components. In vacuum steam stripping, the oils are contacted by steam percolating upwardly in columns or trays of oil at elevated temperatures and subatmospheric pressures.
Many prior art processes of deodorization are discontinuous; i.e. the processes are performed in discrete stages, each completed before the next stage begins. For example, a batch of oil might be degummed and deaerated, then stored before further refining and deodorizing. Oils and fats may also be blended after refining and storage to be subsequently deodorized as a single product.
The first step of the prior art is the removal of water soluble phosphatides, or more particularly, lecithins. The process is known as degumming. The oil is washed with a mixture of water and acid, usually phosphoric, citric or malic (the water and acid is approximately 4% of the weight of the oil) in blenders, high speed mixers and subsequently centrifuged, discharging the phosphatides with the water. As the stability of the oil has been protected by the presence of the lecithins, the oil should be immediately deaerated and dehydrated through contact equipment operating at approximately 150.degree. F. to 175.degree. F. at reduced pressures.
Another step of the prior art is bleaching. Bleaching is the removal of undesirable color from the oil, generally by the use of clays capable of absorbing the color ingredients upon its surface. The mixture of oil and clay is then separated in pressure filters leaving the oil free of the objectionable color. This step is usually performed separately from the other steps.
Following degumming, deaeration, dehydration and bleaching the oil is refined and deodorized. If the oil is a normal vegetable oil, not cotton seed oil, and has not been abused in extraction, transportation or storage, it will contain, at this point, approximately 0.5% removable hydrocarbons with the balance being pure triglycerides. Not accounting for trace materials, the 0.5% will contain about 85% free fatty acids and 15% tocopherol.
Two methods of refining are commonly used: caustic and steam. Caustic refining reacts the free fatty acids with caustic, saponifying the free fatty acid for removal. Steam refining recognizes that most of the free fatty acids are subject to normal distillation and the method uses steam to spring and carry the distillates from the oil. Because of temperature constraints, the latter method must be vacuum stripping and distillation.
For batch type deodorization, the equipment most frequently used for steam stripping deodorization of edible oils, not including deaeration, has been a singular column with a singular vacuum source placed on top of the column. The most common vacuum system is a multiple effect or staged steam eductor system. Oil is retained in the column or recycled through the column while stripping steam is bubbled up through the column for stripping free fatty acids or other odoriferous materials from the oil. The overhead from the column is a mixture of eductor steam, stripping steam, tocopherol, free fatty acids and small amounts of other materials. Processing continues until the desired or economical amount of deodorization has been achieved.
In addition to the separate batch process steps of refining the prior art also includes a semi-continuous deodorization process in which several functions are performed within a singular column. Heating and cooling as well as selective distillation and steam stripping are accomplished concurrently between several trays (really tanks because of their size) within a common column. Present practices frequently allow the oil product to become reaerated after degumming or caustic refining and likewise deaeration procedures are frequently not carried to completion. Hence, subsequent deaeration is performed within the semi-continuous deodorization process. The process holds all stages (large trays) for a particular "batch time" although the "batch time" is not as long as for the batch type deodorization process. Often, less pure oil is produced because a more pure product would require a much longer residence.
In both the batch and semi-continuous processes, the prior art has attempted to decrease the absolute pressure within the column in order to reduce the residence of the oil in the columns. Large amounts of energy are expended to accomplish the low pressure in order to reduce the residence and in order to make the volatile materials spring from the oil more quickly.
Present practice also over-heats the oil all too frequently. In the past, the high temperatures of the column are obtained promptly by Dowtherm.TM. heat exchangers. Exceedingly high skin temperatures of the oil are created in these heat exchangers prior to the oil entering the columns in order to hurry the deodorization to completion. It is well recognized that the longer the oil is subjected to higher temperatures, the more the oil breaks down to produce additional odoriferous materials which must also be removed from the oil. Of course, greater energy use to obtain higher vacuum, higher temperatures or shorter residence reduces the profitability of the process. If longer residence times are permitted, the increase in labor and the decrease in availability of the equipment also reduces profitability.
As can be seen, the prior art methods have several disadvantages. One significant disadvantage is the energy inefficiency of these processes. In order to increase the throughput of the equipment, more and more horsepower has been utilized to increase the vacuum within the steam stripping chambers. As the current practice is not truly continuous, true product to product heat transfer cannot approach its ultimate efficiency as heat transfer is a function of time. Eductor steam has the disadvantage of not yielding its full energy into the production of the low pressure as all of its energy cannot be used in producing dynamic work upon the vapors to be removed. In other words, the eductor steam cannot expend its enthalpy energy upon the system.
Another significant disadvantage of the prior art has been that the methods utilized create environmental problems due to the manner in which the volatile materials are discarded. For deodorization, the quantity of eductor steam plus the stripping steam may equal the mass of the oil being deodorized. Since all the vapors are co-mingled with the steam flow, the total heat lost over head from the liquid oil is extravagant. Likewise, this mixture of hydrocarbon vapor and water vapor present a separation problem with a great deal of the hydrocarbon, when condensed, being sent to local sanitary facilities as this effluent is not acceptable for stream discharge. The BOD of the hydrocarbon water mixture is extremely high with most municipalities making surcharges. Desirable materials are discarded with the eductor and stripping steam which could be marketable if they were to remain uncontaminated. For example, tocopherol is frequently discarded along with fatty acids and water.